A planar light device (30) for emitting light beams, including a light guide plate (31) having a plurality of diffraction grating units (33). The light guide plate includes a light-emitting surface (313), an opposite bottom surface, and a light incident surface (311) disposed therebetween. A light source (32) is disposed adjacent to the light incident surface. The diffraction grating units cover the bottom surface of the light guide plate. Each diffraction grating unit is divided into a first part (A) and a second part (B), the first and second parts having different alignment directions from each other.